The present invention relates to digital communications and more particularly to synchronization of frequency between a receiver and a transmitter.
In an OFDM (Orthogonal Frequency Division Multiplexing) communication system, a channel to be used for communication is divided into subchannels that are orthogonal to one another in the frequency domain. Data is communicated in a series of time domain bursts. To form each time domain burst an IFFT is applied to a group of frequency domain symbols and a cyclic prefix is added to the transform result prior to transmission. Transmission may involve conversion of the transform result to an analog signal, conversion of the analog signal to an intermediate frequency (IF), then upconversion to a desired selectable carrier frequency prior to final amplification and propagation across a transmission medium. Upconversion is typically achieved by mixing the IF signal with a variable frequency oscillator signal. The carrier frequency is varied by varying the oscillator frequency.
On the receiver end, preamplification is followed by downconversion to IF from the carrier frequency, again by mixing with the output of a variable frequency oscillator. The resulting IF signal is typically converted to a baseband digital symbol sequence. The cyclic prefix is removed and an FFT is applied to recover the original frequency domain symbols.
For successful communication, the transmitter and receiver should have a precise shared understanding of the transmission frequency. In the exemplary system described above, this means that the variable frequency oscillators of the transmitter and receiver should be locked to each other. Imprecision with respect to the transmission frequency will cause inaccurate recovery of the OFDM symbols. To maintain system performance, it is desirable to always maintain frequency offset between a transmitter and a receiver to within 1% of the spectral width occupied by a single frequency domain OFDM symbol. When the receiver initially acquires the transmitter frequency, it is desirable that the synchronization system tolerates and corrects as wide as possible a frequency offset between the transmitter and receiver oscillators. This allows the use of much lower cost analog components for the receiver oscillator.
U.S. patent application Ser. No. 09/245,168, filed on Feb. 5, 1999, and entitled SYNCHRONIZATION IN OFDM SYSTEMS discloses various systems and methods for synchronizing the receiver frequency of an OFDM receiver to the transmission frequency of an OFDM transmitter. One such system provides a supplemental cyclic prefix that follows the cyclic prefix used to orthogonalize the frequency domain subchannels. At the receiver end, this supplemental cyclic prefix is correlated to the corresponding time domain symbols within the principle portion of the time domain burst in order to compute a fine frequency offset, that is a fractional component of the frequency offset as measured in OFDM frequency domain symbol widths. Once the fine offset is computed, it may be corrected by use of appropriate control signals to the receiver variable frequency oscillator. This procedure corrects for frequency offsets that are a fraction of a frequency domain symbol width but after this correction the received frequency may still be offset from the transmit frequency by an integer number of frequency domain symbol widths.
The procedure for correction of this integer frequency offset takes advantage of frequency domain structure within each OFDM burst. Each OFDM burst includes regularly spaced training symbols having known predetermined values. The training symbols facilitate estimation of the channel response at the receiver and correction of the integer frequency offset. The integer frequency offset is corrected by finding the frequency alignment that causes the received symbol values at the known training positions to correlate strongly between successive bursts.
It will be appreciated, however, that there is a limit to the acquisition range for this integer frequency offset correction technique. If the frequency offset is greater than the spacing between training symbols, then this offset correction technique may lock to a false alignment that differs from the correct alignment by an integer multiple of the training symbol spacing. The acquisition range is thus   ±      N          2      ⁢                           ⁢      υ      tones where N is the number of frequency domain symbols in a single OFDM burst and v is the number of frequency domain symbols reserved for training.
Consider a millimeter wave application where the operating frequency is 28 GHz and where use of low cost analog components may cause an initial frequency offset of 10 parts per million (ppm) or 280 KHz. A typical value, however, for   N      2    ⁢                   ⁢    υ  may be a small as 4 frequency domain symbol widths. In a representative system where the overall bandwidth of the OFDM system is 6 MHz and where N is 256, this provides an acquisition range of only approximately ±94 KHz or approximately ±3 ppm.
What is needed is a system for OFDM frequency synchronization that can correct for wide offsets that exceed the spacing between training symbols within the frequency domain bursts.